The Secret Life of Mussels

     by Gordon P. Gates

 

I thoroughly enjoyed Trevor Corson's book, The Secret Life of Lobsters. I just wish he had included recipes. Nevertheless, in tribute, here is the first installment of The Secret Life of Mussels.

The blue mussel, Mytilus edulis, is the dominant shellfish species along the coast of Maine, and mussels are plentiful in the subtidal and intertidal zones here in Penobscot Bay.

Blue mussels have been cultivated in Europe for centuries. They are grown on the sea bottom, and on ropes suspended from rafts floating on the ocean surface, and on posts embedded in the intertidal zone.

The post-growing method began in France. The legend is that Patrick Walton, an Irishman shipwrecked on the coast of France in 1235, planted posts in the water to secure nets in an attempt to catch fish or sea birds. That plan didn’t work, but the mussels that grew on the posts were delicious. Thus began an industry that remains today.

These oak posts are 4-5 meters high, and are placed a meter apart in long rows parallel to the shore. The mussels on the posts can be easily thinned and harvested, and France's long expanses of intertidal mud flats make this method of cultivation ideal. There are now over 700 miles of mussel posts along the coast of France.

These mussels posts are called “bouchots,” and the mussels grown on them are given that same name. Such is the cache of the bouchot name that small mussels cultivated in the U.S. are sometimes marketed as “bouchots,” even though they were rope-grown, rather than post-grown.

In July of 2006, the bouchot mussels of the Mont St Michel bay were awarded an Appellation d'origine Controlee (AOC). The AOC designation is bestowed upon a food, usually a wine or a cheese, whose characteristics are uniquely related to its place of origin. The bouchots are the first seafood to be awarded the French “appellation” label.

The modern rope method of growing mussels began on the Mediterranean coast of Spain in l90l. This method has been copied around the world, including here in Maine. The ropes are suspended from large rafts, and the mussels attach to the ropes, where they hang safely above most predators. The shells of these rope-grown mussels are noticeably thinner than their wild brethren.

Mussels are also grown on the bottom. This method is least efficient, and the mussels are vulnerable to bottom-dwelling predators.

Predators -

Of course, we humans are not the only animals that eat mussels. Other animals regularly consume mussels, and have developed unique methods for getting inside the mussel’s protective shell. Sea otters, for example, are well known for using a tool – a rock – to smash open the shells of mussels and other bivalves. Birds also eat mussels. A seagull will snag a mussel at low tide, fly high in the air, and drop the mussel mid-flight onto rocks on the shore, cracking open the shell. Every now and then the gull will miss, which is why mussel shells are sometimes found in the mossy interior of a coastal island.

However, the main mussel predators are marine animals. The starfish, also called a sea star, feeds extensively on mussels. To eat a mussel, the starfish positions itself over the mussel, wraps its strong arms around the mussel’s two shells and slowly pulls them apart. The starfish then feeds in a remarkable way. The starfish turns its stomach inside out, and inserts it through the gap it has created between the shells of the mussel. The stomach secretes digestive enzymes which dissolve the soft mussel body, and the stomach then reabsorbs the digested mussel as food. Thus, the starfish actually digests its food outside its body (called extracorporeal digestion), and the mussel is eaten alive while still in its shell.

Mussels have something of a defense to this predation. In response to the presence of starfish, a mussel can increase the weight of its posterior adductor muscle, which holds the shell closed. It can therefore better resist the starfish’s attempt to pull open the shell. Marine biologists call this an induced response, because it is induced by the presence of a predator. The mussel also has an induced response to the presence of crab predators. A crab will use its claws to crush a small mussel shell to get inside, and mussels in the presence of crabs thicken their shells to avoid this predation. This may be one reason why the shells of rope-grown and bouchot mussels have much thinner shells than wild mussels (another is that tidal and wave pressures lead the mussel to grow a thicker shell).

Another voracious mussel predator is the carnivorous whelk, which is a snail. On the Atlantic coast of the United States, the dogwhelk, Nucella lapillus, primarily feeds upon blue mussels. The dogwhelk is about an inch and a half long, and has a thick whitish shell. To prey upon on a mussel, the whelk drills a small hole through the shell of a mussel, using a rasp-like mouth part (called a radula). This drilling process takes days. The whelk then extends a tube (called a proboscis) into the mussel shell, injects digestive enzymes into the mussel’s soft body, and sucks out the dissolved tissue. Once again, the mussel is eaten alive in its own shell via extracorporeal digestion.

As a child, I would find mussels shells on the beach with one small hole drilled through the shell. The hole was always a tiny, perfect circle. That is the mark, or “predation signature,” of a whelk. The mussel has no reliable defense to this snail drilling attack, but can employ some deterrents. When mounted by a whelk, mussels can open and then slam closed their shells to try to dislodge the snail. Mussels can also use their muscular foot to sweep the surface of their shells, a technique that could remove whelks within reach. However, these tactics do not work much of the time.

When all else fails, the mussel has one more trick. The mussel, and its mussel neighbors, can counterattack using their bysall threads, the strong threads that constitute a mussel’s “beard.” By attaching several byssal threads onto the whelk's shell, the mussel can tether the whelk, thus trapping it. An ensnared whelk may actually be overturned in this fashion, thus stopping the drilling process and sparing the mussel’s life. However, if the whelk is not overturned, it completes its drilling, inserts its proboscis and enjoys its last meal. The whelk is doomed, however, since it is tethered by the strong bysall threads to the shell of its last victim, which is itself firmly attached to a rock. The whelk cannot escape (from either the dead mussel or the irony). The trapped whelk will then starve to death or become an easy meal for a crab. The successful whelk, therefore, must carefully select the spot on the mussel shell to drill its hole. The whelk must find a spot near the top of the mussel shell that is beyond the ambit of the mussel’s foot. And that, by the way, is exactly where you will see the small hole in a mussel shell that you find on the beach.

One way for a mussel to avoid extracorporeal digestion by a starfish or a whelk is to live in brackish water. Those organisms cannot tolerate salinity changes or brackish water. Not surprisingly, mussels thrive in brackish water, where starfish and whelks cannot go. They do not end up there by accident, however. Juvenile mussels can select locations based upon salinity…more on that later.

 

 

 

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